Mapping Northern Ireland’s coastal zone

The shallow nearshore coastal zone plays a critical role in coastal erosion processes, flood risk and ecosystem dynamics, yet it is one of the most difficult environments to survey. This article examines how airborne Lidar bathymetry has been successfully used to survey the full coastal zone of Northern Ireland, representing the first bathymetric Lidar project of this scale undertaken in the United Kingdom.

A Northern Ireland coastal erosion risk management study concluded that there was a lack of consistent nearshore data, required to reliably identify and map areas vulnerable to erosion and flooding. This lack of data was most pronounced in highly dynamic shallow coastal waters, which play a key role in erosion processes but remain difficult to survey.

In these shallow areas, vessel-based multibeam sonar surveys are often constrained by water depth, navigational safety and operational efficiency. As a result, coverage of the land-sea interface is frequently incomplete, leaving an unmapped transition zone between land and water, commonly referred to as the ‘white ribbon’ in coastal surveying. Airborne Lidar bathymetry (ALB) is a proven and efficient technology to address this challenge and close the gap. Using laser pulses, it allows terrain elevations both above and below the water surface to be measured in a single survey operation.

Against this background, the Northern Ireland Department of Agriculture, Environment and Rural Affairs (DAERA) commissioned a project using ALB to map the full coastal zone of Northern Ireland to a target water depth of 10m, covering approximately 700km² of coastal waters. This survey project represents the first ALB mapping project of this scale undertaken in the United Kingdom (UK). Following a tender procedure, the Norwegian company Field was selected to execute the survey.

Survey setup and planning

Field executed the survey by using a fixed-wing aircraft equipped with a CZMIL SuperNova bathymetric Lidar system developed by Teledyne Optech. This sensor was selected for its high laser power, supporting significant depth penetration, combined with advanced waveform processing that helps maintain reliable bottom detection in turbid and optically complex waters. In the context of Northern Ireland’s environmental conditions, this configuration offered the greatest likelihood of achieving useful bathymetric coverage across a substantial proportion of the 0-10m depth range. In addition, a Phase One iXM-RS150F aerial camera was used to acquire 3cm-resolution RGB imagery simultaneously with the Lidar data. The imagery provides valuable contextual information on water clarity, seabed features and coastal morphology, and supports quality control, interpretation and integration of the bathymetric results.

In general, ALB surveys can be executed most efficiently in environments with clear waters and relatively calm wind and wave conditions. In contrast, the coastal waters of Northern Ireland are characterized by quite variable weather conditions, including frequent rainfall and periods of stronger winds, which limit the number of suitable ALB survey windows. Particularly turbidity, commonly expressed as water clarity, is a critical factor for ALB surveys, as suspended sediments in the water column directly reduce laser penetration and achievable depth.

Many nearshore areas in Northern Ireland, specifically the river deltas, have persistently high concentrations of suspended sediments, so it was clear from the outset that reaching the target 10m water depth everywhere along the coastline would be challenging. To address these environmental constraints and maximize the potential survey results, careful attention was given to an advance assessment of local turbidity conditions. Satellite-based turbidity maps were used to assess spatial and seasonal variability in water clarity around Northern Ireland.

Figure 1 shows average turbidity conditions for February and June, derived from satellite observations of the diffuse attenuation coefficient at 490nm (Kd₄₉₀), based on the NOAA Star Ocean Color website. The colour scale ranges from blue (indicating very clear waters with low turbidity) to red (indicating highly turbid conditions with strong light attenuation). Based on both the theoretical depth penetration capability of the CZMIL SuperNova and Field’s practical ALB survey experience, only limited bathymetric penetration of a few metres can typically be expected in red areas, while green areas generally allow depths of around ten metres to be reached. Blue conditions, which are more typical of very clear waters such as those found in parts of the Mediterranean, can enable depth penetration of several tens of metres, but are not observed around the Northern Ireland coast.

Two conclusions can be drawn based on the patterns visible in the maps in Figure 1. First, water clarity along the coast is generally significantly better in June than in February, reflecting typical seasonal differences between winter and summer conditions. In winter, frequent storms generate higher wave energy, stirring up bottom sediments, while increased rainfall and river discharge further elevate nearshore turbidity. Second, the maps reveal a clear geographical variability. The northern coastline generally exhibits clearer conditions, while river estuaries such as Lough Foyle, Belfast Lough and Carlingford Lough remain more turbid throughout the year due to continuous sediment input. These turbidity patterns provided a useful indication of where the target depth of ten metres was likely to be achievable and where coverage would be more limited. On this basis, the main acquisition campaigns were scheduled as far as possible during the summer period, when improved water clarity typically offers the best survey conditions in Northern Ireland.

In addition to sediment-driven turbidity, biological factors such as algal blooms were also considered. Algal concentrations can significantly reduce water transparency, and vary rapidly in both space and time. While difficult to predict in advance, bloom events were monitored during survey execution using near-real-time satellite imagery, including Copernicus ocean colour products. This allowed survey priorities to be adjusted where reduced optical water quality was observed. Tidal conditions were also considered during the survey. Although the tidal range in Northern Ireland is relatively modest, tidal currents can resuspend sediments and cause rapid changes in water clarity, particularly in shallow and confined areas. Periods with reduced tidal currents, such as neap tides, were therefore generally preferred. However, optimal conditions varied locally depending on bathymetry, sediment characteristics and vegetation, requiring flexibility during survey execution.

Survey execution

The airborne survey over Northern Ireland was executed through a series of separate acquisition campaigns (see Figure 2). The full coastal zone was initially surveyed during the summer of 2023, when conditions were most favourable for large-scale data acquisition. As anticipated during survey planning, the bathymetry of several areas, particularly shallow estuaries and persistently turbid nearshore zones, could not be fully covered in a single pass. These areas were therefore addressed through targeted gap-filling campaigns in 2024, in some cases requiring multiple reflights to improve bathymetric coverage and point density. The final reflights were completed during a mobilization in May 2025.

Product creation

The acquired Lidar and imagery data was processed to generate cleaned and classified topo-bathymetric point clouds, digital elevation models and depth contours, as well as high-resolution orthophotos. Together, these products form a comprehensive and highly detailed dataset of the coastal zone of Northern Ireland.

In addition to bathymetry, the delivered products include gridded Lidar return intensity and a separate classification of marine vegetation, both of which add significant value to the dataset. Lidar intensity supports the interpretation of seabed characteristics and spatial variability, while vegetation mapping provides insight into the distribution of submerged habitats such as kelp and seagrass.

Visualizing the coastal zone

The dataset forms a continuous topo-bathymetric elevation model, capturing elevations above the water surface and bathymetric depths extending beyond ten metres in many areas.

In some cases, local environmental conditions imposed limitations. For example, in parts of Lough Foyle (see Figure 3), particularly near the mouth of the River Foyle and around Londonderry, bathymetric coverage remains incomplete due to persistently high turbidity, which limited bottom detection despite multiple acquisition attempts. Even so, the resulting digital elevation models clearly depict the transition from dry land through the intertidal zone into subtidal channels, including sandbanks and channel features.

Under more favourable conditions, the combined Lidar and imagery data revealed detailed underwater topography, with bathymetric penetration extending well beyond the 10m depth contour across much of the area (see Figure 4). The zone between the coastline and the sonar limit, the so-called ‘white ribbon’ area, is largely inaccessible to multibeam surveys due to shallow water depths and navigational constraints. Figure 5 clearly illustrates how airborne Lidar bathymetry fills this gap, providing continuous coverage across the land-sea interface and demonstrating the strong complementarity between multibeam sonar and airborne bathymetric Lidar for comprehensive coastal mapping.

Survey results

Following processing and quality control, approximately 85% of the Northern Ireland coastal zone was successfully mapped to depths of 10m or greater using airborne Lidar bathymetry. In clearer water areas, bottom detection frequently exceeded 25m, while in more turbid environments, such as Strangford Lough, achievable depths were typically limited to around 8m. A small number of localized data gaps remain, primarily within major estuaries and sea loughs, as well as in isolated rocky or kelp-dominated nearshore sections. In these areas, persistently elevated turbidity, strong tidal sediment transport, dense marine vegetation and/or dark, low-reflectance seabed materials limit laser penetration and reflection, even after repeated acquisition attempts under different environmental conditions.

Independent validation was carried out by the UK Hydrographic Office prior to final acceptance. Across all surveyed regions, the bathymetric data met the requirements of IHO S-44 Order 1b. The UKHO assessment confirmed good internal consistency and seamless integration across the land-sea interface and with adjacent multibeam sonar datasets.

Taken together, the results represent the most complete and consistent topo-bathymetric dataset acquired for the Northern Ireland coast to date. By effectively bridging the gap between terrestrial Lidar coverage and deeper-water multibeam surveys, the ALB dataset provides a robust foundation for coastal management, charting, engineering and environmental applications. This project further demonstrates that airborne bathymetric Lidar surveys can consistently deliver reliable, high-quality data across extensive coastal zones, even under challenging environmental conditions. By evidencing robust performance in areas with variable weather and water clarity, the project helps reduce technical and operational uncertainty, thereby lowering the barrier for future coastal Lidar bathymetry initiatives in the UK and in other regions facing similar constraints.

Further reading

Baseline Study and Gap Analysis of Coastal Erosion in Northern Ireland, by the Department for Infrastructure (DfI) and the Department of Agriculture, Environment and Rural Affairs (DAERA), https://www.infrastructure-ni.gov.uk/publications/baseline-study-and-gap-analysis-coastal-erosion-risk-management-ni